This invention relates to a lightning arrester, and more particularly to a lightning arrester for protecting power station equipment from damage caused by lightning disturbances.
With the conventional lightning arrester, a plurality of ZnO base nonlinear resistor elements are connected in series in accordance with the level of the rated voltage to form a valve block. One end of this value block is connected to a line coupled to station equipment requiring protection. The other end of the valve block is grounded. In other words, the conventional lightning arrester is not provided with a spark gap or space. The reson why the spark gap need not be provided is that where voltage impressed in a ZnO base nonlinear resistor element has a lower level than prescribed, then substantially no current flows through the element; and conversely where the voltage exceeds the prescribed level, the ZnO base nonlinear resistor element displays such impressed voltage--current characteristic that a pulsating current flows through the resistor element.
With the known lightning arrester constructed as described above, the spark-starting voltage of the valve block is proportional to the number of series-connected nonlinear resistor elements. In other words, the smaller the number of the resistor elements, the more reduced the spark-starting voltage of the prior art lightning arrester. The reason for this is that the total resistance of the valve block falls more, as the resistor elements decrease in number; under the normal condition, more leak current flows through the valve block, raising the temperature of the valve block. This temperature rise leads to the passage of more leak current, until the valve block is damaged, thus disabling the lightning arrester.
With another type of known lightning arrester, a pair of spatially arranged electrodes are connected in series to the valve block to prevent its thermal breakdown. A space provided between the paired electrodes prevents leak current from flowing through the nonlinear resistor elements of the valve block under the normal condition, thereby saving the resistor elements or the valve block from a thermal breakdown. The level of spark-starting voltage occurring in the above-mentioned inter-electrode space or spark gap varies with the waveform of abnormal surge voltage impressed on a lightning arrester. In other words, where the duration of the wave front of surge voltage is shortened, and a steep rise of lightning current results, then the level of spark-starting voltge is raised, thus desirably reducing the station equipment-protecting property of a lightning arrester.